(1) Field of the Invention
The present invention relates to wireless networks and, in particular, to a variety of techniques for the deployment and operation of access points in wireless networks to improve capacity and geographic isolation.
(2) Description of the Related Art
Wireless networks typically employ egress/access devices, commonly referred to as access points, which form points of presence for client radio devices. An access point may act alone in its function but is often deployed in an array or cellular structure with predictable and overlapping coverage from cell to cell. Client devices act as end-points for telemetry and data transferred to and from access points, or as processing points for the telemetry. In a conventional wireless network having multiple access points, a client device will typically associate with the access point for which it experiences the most favorable signal-to-noise ratio (SNR). The client will then attempt to remain associated with that access point for as long as possible (e.g., by tuning down the data transfer rate).
To avoid interference between adjacent access points, conventional wireless networks often employ different channels within the RF band of interest for different access points, e.g., channels 1, 6, and 11 in the RF band associated with IEEE 802.11, the set of standards relating to wireless local area networks. Careful control of the signal intensities for adjacent access points is also used to reduce the likelihood that the access points will interfere with each other.
In a wireless network based on IEEE 802.11 or similar technologies, it is desirable that client devices be able to roam seamlessly from one access point to another. In some applications, it is also desirable to provide and support a variety of functionalities including, for example, independent data paths, multiple data types, independent user permissions, and independent security protocols allowing or restricting access or content based on geographic or venue locations within an environment which may offer little restriction to radio frequency propagation. For example, it may be desirable to enable a user at a venue with the proper permissions to roam from a public area such as a hallway or lobby into a meeting room or convention area. In these new areas the user would then have access to data and permissions not allowed or available in the public area. The capability of restricting the area of influence or usability by defining strict geographic boundaries, i.e., geographic isolation, enhances or enables a wide variety of services such as, for example, E-911, Point location (i.e., a “You Are Here” service), billing by location, traffic management, data security, access control, etc.
Geographic isolation may be conventionally achieved by restricting the broadcast power of the transmitting access point or base station, and in some circumstances the transmitting power of the client devices. In some applications the Effective Isotropic Radiated Power (EIRP) of both the access point and client device may be restricted. This approach is highly effective in large open areas but breaks down in confined areas such as inside buildings or dense urban environments in which “canyon effects” tend to deduct signal.
In some applications, the attenuation presented by structures in the environment may not present a substantial barrier to signal propagation. This may be especially true, for example, in conference or office environments that may only be separated by glass, or thin, movable partitions. It is often not technically feasible to “dial down” the power of a transceiver to the point where it would continue to be useful in its intended area without transmitting beyond such barriers. In addition, reducing the transmission power of access points increases areas of shadow (or signal detected from other access points), while decreasing the ratio of signal to noise. These are both undesirable results in that they increase the likelihood that a client device might roam to an out-of-area access point. And even where this technique may be used successfully, it can be easily defeated by the use of relatively hi-gain antennas on client devices that enable reaching far beyond the intended area of geographic isolation.
Accurate determination of the location of client devices may also be used to achieve the goals associated with geographic isolation. That is, if the position of a client device is known within an environment, access to services may be controlled on that basis. Presently, wireless systems and devices rely on averaged signal strength from a known source point for location telemetry. The accuracy of the location can be improved upon, by a process known as triangulation. Triangulation is a process by which the location of a radio transmitter can be determined by measuring either the radial distance, or the direction, of the received signal from three different points. For example, the distance to a cell phone may be determined by measuring the relative time delays of the normal communications signal from the phone to three different base stations. Signal strength measurements in combination with triangulation have proven to be quite accurate in open environments. However, closed environments such as building interiors and dense urban areas present conditions which seriously degrade the efficacy of such techniques.
That is, the combination of reflection, refraction, multi-path, and signal absorption in such environments form complex boundary conditions making position predictions based on signal strength and triangulation tricky and often inaccurate. Methods to correct for these effects involve highly complex modeling and mapping of signal levels in the environment. And unfortunately, this time consuming and expensive “correction” falls apart if even a small change occurs from the baseline mapping. These small changes include thing like a door opening or closing, a curtain being opened exposing a reflective pane of glass, or even something as innocuous as the variable flow of water in plumbing.
Another conventional approach to determining the location of client devices is accomplished using global positioning systems (GPS) technologies. Unfortunately, such technologies are not always reliable inside buildings or in dense urban environments in that the reach of GPS equipment is limited by the attenuation caused by surrounding structures. GPS solutions also involve the use of secondary equipment, increasing system costs and introducing an additional point of failure.
In view of the foregoing, it is desirable to provide improved techniques for deploying wireless access points, base stations and the like.